As studies have shown, exposing your body to G forces (high or low) has a dramatic effect on bone density.
At rest, your body experiences 1G of weight, which is the normal force of gravity on Earth.
Astronauts in outer space who float around all day at 0G, experience dramatic losses in bone density in just a few short days.
Exercises such as squats and deadlifts on the other hand, expose you to anywhere from 2-3G depending on your strength.
Skateboarders landing jumps can experience as much as 10-20G depending on the height.